1. Field of The Invention
The subject invention relates to lighting systems, and more particularly, to an improved array structure for light-emitting diodes used as illumination sources.
A light-emitting diode (LED) is a type of semiconductor device, specifically a p-n junction, which emits electromagnetic radiation upon the introduction of current thereto. Typically, a light-emitting diode comprises a semiconducting material that is a suitably chosen gallium-arsenic-phosphorus compound. By varying the ratio of phosphorus to arsenic, the wavelength of the light emitted by a light-emitting diode can be adjusted.
With the advancement of semiconductor materials and optics technology, light-emitting diodes are increasingly being used for illumination purposes. For instance, high brightness light-emitting diodes, based on Aluminum Indium Gallium Phosphide and Indium Gallium Nitride technologies, are currently being used in automotive signals, traffic lights and signs, large area displays, etc.
2. Description Of The Related Art
In many of the above-noted applications, multiple light-emitting diodes are connected in an array structure so as to produce a high amount of lumens. FIG. 1 illustrates a typical arrangement of light-emitting diodes D(1)-D(n) connected in series. A power source 1 deliver a current signal to the light-emitting diodes via a resistor R1, which controls the flow of current in the diodes. Light-emitting diodes which are connected in this fashion usually lead to a power supply with a high level of efficiency and a low amount of thermal stress.
Occasionally, an LED may fail. The failure of an LED may be either an open-circuit failure or a short-circuit failure. For instance, in short-circuit failure mode, light-emitting diode D(2) acts as a short-circuit, allowing current to travel from light-emitting diode D(1) to D(3) through light-emitting diode D(2) (which does not generate light). On the other hand, in open-circuit failure mode, light-emitting diode D(2) acts as an open circuit, and, as such, causes the entire array of FIG. 1 to extinguish.
In order to address this situation, other arrangements of light-emitting diodes have been proposed. For example, FIG. 2A illustrates another typical arrangement of light-emitting diodes which consists of multiple branches of light-emitting diodes 10, 12, 14 and 16, connected in parallel. Each branch comprises light-emitting diodes connected in series as in FIG. 1. In FIG. 2A, branch 10 comprises light-emitting diodes D1(1) to D1(n), connected in series; branch 12 comprises light-emitting diodes D2(1) to D2(n); branch 14 comprises light-emitting diodes D3(1) to D3(n); and branch 16 comprises light-emitting diodes D4(1) to D4(n). Power source 2 provides a current signal to the branches 10, 12, 14 and 16 via a resistor R2.
Light-emitting diodes which are connected in this fashion have a higher level of reliability than light-emitting diodes which are connected according to the arrangement shown in FIG. 1. In open-circuit failure mode, the failure of a light-emitting diode in one branch causes all of the light-emitting diodes in that branch to extinguish, without significantly affecting the light-emitting diodes in the remaining branches. However, the fact that all of the light-emitting diodes in a particular branch are extinguished by an open-circuit failure of a single light-emitting diode is still an undesirable result. In short-circuit failure mode, the failure of a light-emitting diode in a first branch may cause that branch to have a higher current flow, as compared to the other branches. The increased current flow through a single branch may cause the remaining light-emitting diodes to luminesce at a different level than the light-emitting diodes in the remaining branches. This is also an undesirable result.
Still other arrangements of light-emitting diodes have been proposed in order to remedy this problem. For example, FIG. 2B illustrates another typical arrangement of light-emitting diodes, as employed by lighting systems of the prior art. The arrangement of FIG. 2B is substantially similar to that of FIG. 2A, with the exception that shunts are connected between adjacent branches of light-emitting diodes. In particular, shunt 4 is arranged between the light-emitting diodes D1(1)/D1(2), D2(1)/D2(2), D3(1)/D3(2) and D4(1)/D4(2) and connects the branches 10, 12, 14 and 16 to each other. Shunts 5 and 6 are similarly arranged between respective light-emitting diodes in the branches 10, 12, 14 and 16, and connect the branches to each other.
Light-emitting diodes which are connected in this fashion have a still higher level of reliability than light-emitting diodes which are connected according to the arrangements shown in either FIGS. 1 or 2A. This follows because, in an open-circuit failure mode, an entire branch does not extinguish because of the failure of a single light-emitting diode in that branch. Instead, current flows via the shunts to bypass the failed light-emitting diode.
However, in the short-circuit failure mode, a light-emitting diode which fails has no voltage across it, thereby causing all of the current to flow through the branch having the failed light-emitting diode. For example, if light-emitting diode D1(1) short circuits, current will flow through the upper branch. Thus, in the arrangement shown in FIG. 2B, when a single light-emitting diode short circuits, the corresponding light-emitting diodes D2(1), D3(1) and D4(1) in each of the other branches will also be extinguished.
The arrangement shown in FIG. 2B also experiences other problems. For example, in order to ensure that all of the light-emitting diodes in the arrangement have the same brightness, the arrangement requires that parallel-connected light-emitting diodes have matched forward voltage characteristics. For example, light-emitting diodes D1(1), D2(1), D3(1) and D4(1), which are parallel connected, must have tightly matched forward voltage characteristics. Otherwise, the current signal flow through the light-emitting diodes will vary, resulting in the light-emitting diodes having dissimilar brightness.
In order to avoid this problem of varying brightness, the forward voltage characteristics of each light-emitting diode must be tested prior to its usage. In addition, sets of light-emitting diodes with similar voltage characteristics must be culled into tightly grouped sets (i.e., sets of light-emitting diodes for which the forward voltage characteristics are nearly identical). The tightly grouped sets of light-emitting diodes must then be installed in a light-emitting diode arrangement in parallel to each other. This culling process is costly, time consuming and inefficient